This invention relates to radioactive well logging and more particularly to an improved method for determining the fluid saturation characteristics of subsurface formations surrounding a borehole from pulsed neutron capture logs which have been recorded by the log-inject-log technique.
Theoretically, the hydrocarbon and water saturations of a subsurface formation can be represented by the following expression: EQU .SIGMA..sub.formation =.SIGMA..sub.rock (1-.phi.)+.SIGMA..sub.fluid .phi.(1) EQU .SIGMA..sub.fluid =.SIGMA..sub.oil S.sub.oil +.SIGMA..sub.water S.sub.water ( 2) EQU S.sub.oil =1-S.sub.water ( 3)
where,
.SIGMA. represents macroscopic absorption cross section, PA1 S represents saturation (or volume fraction of a particular fluid in place), and PA1 .phi. represents porosity.
In rewriting equation (1) in terms of equations (2) and (3), the macroscopic thermal neutron absorption cross section of the formation can be written: EQU .SIGMA..sub.f =(1-.phi.).SIGMA..sub.r +.phi.[S.sub.w .SIGMA..sub.w +.SIGMA..sub.h (1-S.sub.w)] (4)
Of the five variables on the right-hand side of equation (4) required for the determination of hydrocarbon saturation, the macroscopic absorption cross sections of rock matrix (.SIGMA..sub.r), hydrocarbon (.SIGMA..sub.h), and water (.SIGMA..sub.w) are known or estimatable, while the porosity (.phi.) and macroscopic absorption cross section of the formation (.SIGMA..sub.f) must be measured within the borehole.
In many instances the macroscopic absorption cross section of the rock matrix (.SIGMA..sub.r) cannot be known or estimated with good accuracy. Strongly absorbing trace elements are usually too abundant in the rock matrix to permit an accurate determination of .SIGMA..sub.r from the rock's major constituents. The log-inject-log technique has been utilized to overcome this problem. The formation is first flushed with a brine of known salinity and therefore known macroscopic absorption cross section .SIGMA..sub.w1. A pulsed neutron capture log is then recorded. The formation is thereafter flushed with a brine of different salinity and cross section .SIGMA..sub.w2. A second pulsed neutron capture log is recorded. If both flushings are sufficient to achieve residual hydrocarbon saturation in all zones of interest, then the logged cross sections, .SIGMA..sub.log 1 and .SIGMA..sub.log 2 can be written: EQU .SIGMA..sub.log 1 =(1-.phi.).SIGMA..sub.r +.phi.[S.sub.w .SIGMA..sub.w1 +.SIGMA..sub.h (1-S.sub.w)] (5)
and EQU .SIGMA..sub.log 2 =(1-.phi.).SIGMA..sub.r +.phi.[S.sub.w .SIGMA..sub.w2 +.SIGMA..sub.h (1-S.sub.w)] (6)
Subtracting equation (3) from equation (1) yields: EQU .phi.S.sub.w =(.SIGMA..sub.log 1-.SIGMA..sub.log 2)/(.SIGMA..sub.w1 -.SIGMA..sub.w2) (7)
Equation (7) is the conventional log-inject-log expression. With porosity and water macroscopic cross-sections known or measured, the recordings of .SIGMA..sub.log 1 and .SIGMA..sub.log 2 will allow the computations of water saturation corresponding to residual oil saturation, independent of .SIGMA..sub.r.
For a more detailed understanding of the log-inject-log technique reference may be had to U.S. Pat. Nos. 3,748,474; 3,757,575; 3,812,353; and 3,825,752.
Various logging tools are available in the art for measuring thermal neutron capture cross section as a function of depth within the borehole. One such tool is the pulsed neutron logging tool. This tool provides a pulsed neutron log indicative of the time required or the rate at which thermal neutrons emitted by the tool are captured or absorbed by the formation material. This log indicates the macroscopic absorption cross section of the formation, referred to in the well logging art as the thermal neutron capture cross section. One such pulsed neutron log is provided by Schlumberger, Limited of New York, New York under the tradename THERMAL DECAY TIME LOG. Another such pulsed neutron log is provided by Dresser Industries, Inc. of Houston, Texas, under the tradename NEUTRON LIFETIME LOG.